Conventional Duct Assemblies
In many gas handling applications it is necessary to carry heated corrosive gases through suitable conduits. One type of large scale gas conduit system is found in fossil fuel electric power generating plants. The combustion process flue gases are carried between the boiler and the stack through large ductwork that frequently also connects a conventional scrubber, precipitator, and reheater between the boiler and the stack. The scrubber removes sulfur dioxide, the precipitator removes much of the small particulate matter, and the reheater increases the temperature of the flue gases which have become cooled by the scrubber.
Not all of the particulate matter is removed from the gas in all parts of the ductwork. Over a period of power plant operation, fly ash particulate matter settles out of the flue gas stream and coats the inside of the duct. The generally horizontal bottom portions of the ductwork may have to support the weight of tens of centimeters of such fly ash.
Reheating the flue gases with the reheater helps reduce the amount of corrosive condensate. The quantity of corrosive condensate produced decreases with increased temperature. The temperatures of the reheated flue gases may range between about 60.degree. C. (140.degree. F.) and about 82.degree. C. (180.degree. F.) or more.
Such flue gas ductwork is conventionally fabricated from steel plate that typically has a thickness of about 0.635 cm. (0.25 in.) or more. The ductwork is supported from the ground by suitable conventional structural steel support members. The outside of the ductwork may be insulated to reduce heat loss between the boiler and the stack and to thereby maintain the gas temperature as high as possible to reduce condensation in the system.
It is generally intended that fossil fuel electric power generating plants be operated, almost continuously, for 30 or 40 years. Depending upon actual operating conditions (including how often the scrubber, precipitator, and reheater are in or out of service), degradation of the steel ductwork is generally expected to occur. Such ductwork typically suffers acidic and halogen corrosion damage which may eventually create holes in the steel plate of the ductwork and which must be repaired.